Fix file descriptor leak

[AspenJames]

Fixes file descriptor leak as described in #253

Reproduced by modifying _example/simple-echo/main.go, looping the body of the main function in order to observe multiple calls to prompt.Input(). "Exit" added for convenience:

diff --git a/_example/simple-echo/main.go b/_example/simple-echo/main.go
index 12b1bb4..b02199f 100644
--- a/_example/simple-echo/main.go
+++ b/_example/simple-echo/main.go
@@ -12,17 +12,23 @@ func completer(in prompt.Document) []prompt.Suggest {
 		{Text: "articles", Description: "Store the article text posted by user"},
 		{Text: "comments", Description: "Store the text commented to articles"},
 		{Text: "groups", Description: "Combine users with specific rules"},
+		{Text: "exit", Description: "Stop the loop"},
 	}
 	return prompt.FilterHasPrefix(s, in.GetWordBeforeCursor(), true)
 }
 
 func main() {
-	in := prompt.Input(">>> ", completer,
-		prompt.OptionTitle("sql-prompt"),
-		prompt.OptionHistory([]string{"SELECT * FROM users;"}),
-		prompt.OptionPrefixTextColor(prompt.Yellow),
-		prompt.OptionPreviewSuggestionTextColor(prompt.Blue),
-		prompt.OptionSelectedSuggestionBGColor(prompt.LightGray),
-		prompt.OptionSuggestionBGColor(prompt.DarkGray))
-	fmt.Println("Your input: " + in)
+	for {
+		in := prompt.Input(">>> ", completer,
+			prompt.OptionTitle("sql-prompt"),
+			prompt.OptionHistory([]string{"SELECT * FROM users;"}),
+			prompt.OptionPrefixTextColor(prompt.Yellow),
+			prompt.OptionPreviewSuggestionTextColor(prompt.Blue),
+			prompt.OptionSelectedSuggestionBGColor(prompt.LightGray),
+			prompt.OptionSuggestionBGColor(prompt.DarkGray))
+		if in == "exit" {
+			break
+		}
+		fmt.Println("Your input: " + in)
+	}
 }

Each call to Input will leak another file descriptor, seen with lsof -p <GO PID> | grep 'dev\/tty' | wc -l:

Here you can see the count of /dev/tty file descriptors increasing:

Calling syscall.Close(t.fd) prevents this leak. After this fix the count of file descriptors is constant: